The primary investigator is an MD/PhD trained infectious diseases physician with an interest in understanding enzymes that generate diversity in host-pathogen interactions. In the proposed work, the PI aims to bring his prior experience in enzyme mechanisms and develop new training through virologic experiments and immunologic studies. A remarkable group of enzymes, the polynucleotide cytidine deaminases of the AID/APOBEC family, play both constructive and destructive roles in struggle against HIV. On one hand, deamination by the family member APOBEC3G interferes with the integrity of the pathogen genome. In turn, HIV has evolved the lentiviral protein Vif as an evasive means to counteract human APOBEC3G. Infection with HIV is also associated with immune activation, which can result in increased expression of a B-cell specific deaminase family member, AID. AID physiologically serves as the chief catalyst governing antibody diversity through the introduction of targeted uracil lesions in antibody variable genes or switch regions which ultimately result in higher affinity antibodies of altered isotype. Aberrant regulation and expression of AID has increasingly been associated with Non-Hodgkins lymphoma, the leading AIDS-defining malignancy in HIV infected patients. Despite the importance of these cytidine deaminases, little is known about the nature of their interaction with their nucleic acid targets. This proposal addresses the hypothesis that the molecular interactions that lead to catalysis and binding of nucleic acids are critical determinants of their proper physiologic function. The studies aim to decipher and perturb these molecular interactions. Initially, structure-based hypotheses will be used to localize the protein determinants of sequence preference and resolve the mode of binding to the nucleic acid backbone. By utilizing novel loop graft mutant enzymes with altered sequence preference, the impact of perturbed sequence specificity on retroviral restriction (APOBEC3G) or antibody diversity and chromosomal translocations (AID) will be explored. To understand catalysis by AID/APOBEC enzymes, nucleoside analogs will be introduced into oligonucleotides via chemical or chemoenzymatic methods and are used to characterize the kinetics of deamination and the inhibition of pro-oncogenic AID activity. Taken together, a full characterization of the AID/APOBEC-nucleic acid complex - binding and catalysis - will provide a molecular basis for the action of this important enzyme family in vitro and in vivo. Through mentored training, the PI will develop the broad based research skills necessary to examine biological and biochemical aspects of diversity generation in host-pathogen interactions upon an ultimate transition to independence. RELEVANCE The immune system uses enzymes that catalyze the targeted deamination of cytosine to restrict HIV and make high affinity antibodies, though the same mechanisms can potentially be pro-oncogenic. This proposal aims to biochemically define and biologically perturb the molecular interactions that allow polynucleotide cytidine deaminase enzymes of the AID/APOBEC family to protect against infection.